fix: accept long JSON strings in ChatTemplateConfig.from_json

[MohammadHijjawi97]

Motivation

ChatTemplateConfig.from_json is documented to accept "a JSON file or JSON string". It first tries open() on the argument and only falls back to parsing it as JSON on FileNotFoundError. Opening a JSON string as a path can raise other OSErrors, and these are turned into ValueError('Invalid input. Must be a file path or a valid JSON string.') even though the input is valid JSON:

• Windows: EINVAL for any JSON string, since it contains quotes and colons (reproduced);
• Linux: ENAMETOOLONG can be raised once the string exceeds the file-name length limit, e.g. a config with a long meta_instruction.

import json
from lmdeploy.model import ChatTemplateConfig
ChatTemplateConfig.from_json(json.dumps({'model_name': 'x', 'meta_instruction': 'You are a helpful assistant. ' * 12}))
# ValueError: Invalid input. Must be a file path or a valid JSON string.

Modification

Read the argument as a file only when os.path.isfile() is true; otherwise parse it as a JSON string. Invalid input still raises a ValueError (json.JSONDecodeError). Added test_chat_template_config_from_json to tests/test_lmdeploy/test_model.py, covering a long JSON string and a JSON file.

BC-breaking (Optional)

No.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
4. The documentation has been modified accordingly, like docstring or example tutorials.